1. Field of the Invention
The present invention relates to a webbing retractor which structures a seat belt device for a vehicle.
2. Description of the Related Art
There are webbing retractors equipped with a spool (take-up shaft) on which webbing is taken-up, an acceleration detecting mechanism having an acceleration sensor detecting the acceleration of a vehicle, and a pretensioner mechanism forcibly rotating the spool in a webbing take-up direction at the time of an emergency of a vehicle (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 8-133015).
In this webbing retractor, the pretensioner mechanism is disposed at one side of the frame, whereas the acceleration detecting mechanism is disposed at the other side of the frame, such that the balance of the arrangement of the mechanisms (the balance in terms of space) at the one side and the other side of the frame is good.
In such a webbing retractor, there are cases in which other mechanisms requiring a certain amount of space for placement are added, such as, for example, a motor retractor mechanism which takes-up the webbing at the time when a collision of the vehicle is predicted, or the like. In such cases, for example, the balance of the placement of the mechanisms can be made to be good by placing this other mechanism at one side of the frame, and placing both the pretensioner mechanism and the acceleration detecting mechanism at the other side of the frame.
The pretensioner mechanism described above has, for example, a pinion provided coaxially with respect to the take-up shaft, a cylinder provided in the vicinity of the pinion, and a piston provided movably within the cylinder. At the time of an emergency of the vehicle, due to gas being supplied to one side of the piston within the cylinder and the piston moving toward the other side, a rack formed at the piston meshes-together with the pinion, and rotates the pinion in the take-up direction. A clutch is provided at the pinion. When the pinion rotates, the pinion and the take-up shaft are connected via the clutch, and the take-up shaft is rotated in the take-up direction.
Moreover, in the pretensioner mechanism, generally, a small gas venting hole is formed in the flange portion of the piston, and the high-pressure gas supplied to the interior of the cylinder is exhausted via this gas venting hole. In this way, it is possible to prevent the gas pressure within the cylinder from rising extremely.
However, in a case in which both the pretensioner mechanism and the acceleration detecting mechanism (the acceleration sensor and the like) are disposed at the same side of the spool as described above, some type of measure must be taken in order for the gas, which is exhausted from the gas venting hole of the piston, to not affect the acceleration detecting mechanism.